For the napkin calculation: On disk, the IEntry is 38Bytes. Alas, writes occur always in (the ssd internal) blocksize. So, essentially (assuming 4096 byte blocksize, which is quite optimistic), we have a write efficiency of less than 1 percent.

A good firmware in the ssd could avoid needing a new block for the write, if all bits are changed in teh same direction by the new data. This is defeated by the binary search in the index buckets. The entries must be sorted, and the number of used entries is tob updated. So, for each new entry the whole (internal) block has to be remapped. Therefore an erasure block must be erased after about  1m/4k index writes.

It seems, venti in its current form is a ssd killer, if they are used for the isects.